Probing the deep structure of atoms, molecules, and solids often requires X-rays. The energies and wavelengths of X-ray light are ideal for examining electronic spin properties, chemical details, and interactions, where no other type of light can reach. For this reason, there is a lot of interest in developing X-ray lasers. While we've managed to turn some particle accelerators into free-electron X-ray lasers, benchtop X-ray lasers would make advanced imaging much more approachable.

Now, researchers have developed a device that starts with an infrared laser and converts it into a beam of higher-intensity photons. The new device isn’t the same as a laser, in that it emits across a broad spectrum of wavelengths. But the light it produces is coherent, and, most importantly, it extends into X-rays without requiring a particle accelerator.

As described in a new Science paper, Tenio Popmintchev et al. directed short pulses from an infrared laser onto atoms of several gasses held under high pressure. The complex interaction between the infrared photons and the electrons within the atoms produced a broad spectrum of light, ranging from ultraviolet up to X-rays. The emitted light was coherent, meaning the photons travel together in a correlated fashion, and came in very short pulses of intense light.

The particular technique the researchers used is known as high-harmonic generation (HHG), akin to the high-pitched squeak from a string in a musical instrument that sometimes accompanies lower notes. The difference is that while a musical instrument may produce a dozen harmonics, HHG in high-pressure gas can make thousands of harmonics, and the "notes" are frequencies of light. In fact, so many frequencies were made in this experiment that they appear to be a continuum instead of sharp individual "notes," so the authors refer to it as a supercontinuum.

HHG is a general reaction of atoms when exposed to ultrafast laser light. While infrared light is insufficiently energetic to ionize the atoms, the electric field associated with a short pulse of light whips an electron back and forth. As the electrons settle down, new photons are emitted. Additionally, the electrons interact directly with the wave aspects of the light, something known as quiver motion.

To create X-ray light using HHG, the researchers used femtosecond (10-15 second) pulses from an infrared laser, directed onto a container of gas (helium, neon, argon, or nitrogen). The container itself is a waveguide, a chamber with a shape, dimensions, and electrical properties that shape the behavior of the light wave. The waveguide geometry and the high pressures in the gas together give rise to the HHG. In this case, the researchers found an optimal pressure in helium of about 35 atmospheres; above that, the atom-atom interactions broke up the coherence of the emitted X-ray light.

The authors devoted a lot of the Science paper to showing that the X-ray light was actually coherent, which highlights how challenging this sort of short-timescale physics can be in practice. They also discussed the difficulty of comparing their experimental results to some aspects of the theoretical models for this behavior—they hope their working hardware will improve the models, since this is a key step to constructing even more energetic X-ray lasers.

That point is important, since the researchers predict they can extend their method to produce higher-energy bursts of light over even shorter time scales (zeptoseconds, or 10-21 second, which I admit I bring up largely to use the term "zeptosecond"). In all, this experiment is a significant advance in benchtop X-ray lasers.

The articles says the X-Rays are coherent, but it doesn't operate using the principles of a laser.

They can tune the energy of the X-Ray photons up to ~1.6keV. X-Ray photoelectron spectroscopy uses 1.2 and 1.4 keV X-Rays for the Mg and Al anodes. However, they have such a broad range of photon energies at higher energies, so I guess you'd need a monochromater to do anything like classical UV or XPS studies.

Also, you typically need to use high energy X-Rays/UV light under high vacuum (10^-8 to 10^-10 torr) because they will interact with gases in addition to your sample.